In recent years, the renewable energy generation market has called for the development of increasingly large wind turbines to meet increasing demands. This development means the appearance of technical problems, derived from the enormous structures built, in particular in the wind blades, largely built in fibre-reinforced plastic which require being joined to the wind turbine hub.
However, fibre-reinforced plastics do not work well in high-stress concentration conditions, such as those which appear in the joining of the blade root to the hub since, for certain blade sizes, problems of fatigue usually appear, mainly due to the large quantity of blade load cycles.
Wind turbine blades are generally laminated composite material, which is formed by several layers of reinforced material joined by a resin and cured to consolidate the blade. To guarantee the fixing of each one of the blades to the hub, the solution is generally adopted of incorporating internally threaded metal inserts, which are introduced during blade manufacture, in combination with fixing screws of the hub which thread in these inserts of the blade establishing a connection of the hub to the blade.
Document U.S. Pat. No. 4,915,590 is an example of this type of joint, since it shows several inserts internally embedded in the blade root, in parallel disposal to the blade shaft, wherein each insert incorporates a longitudinal threaded bore designed to receive a screw of the hub, so that the screws thus introduced in the inserts establish the fixing between the blade and the hub.
Document U.S. Pat. No. 7,530,168 relates to a manufacturing method of a wind turbine blade and the blade thus obtained, detailing that the blade root comprises a plurality of inserts or bushings provided with an internal threaded sector designed to receive the screws that connect the blade to the hub. In this case it has been provided that the inserts have two sections, or that they are formed by two joined portions, wherein one of them is of cylindrical tubular configuration and integrates the threaded sector and the other is of chamfered tubular configuration with progressively decreasing section until its distal end.
This document discloses the process of forming the blade root comprising the phases of: providing a holder having spaced recesses designed to house the inserts, placing a first layer of fibre on the holder, locating the inserts in said recesses, and then providing additional fibre layers on the inserts, ending with the fibre consolidation stage.
Document ES 2382439 shows the type of inserts described above, wherein they appear separated on a circular arc of the blade, and also spacing elements situated between the inserts are observed, with the spacers being connected with the inserts by one external laminate and another internal laminate.
Furthermore, other joining solutions between blade and hub are known, such as that shown in US 2009/0263250, wherein it is possible to observe that the blade root has transversal pins disposed in radial or transversal position to the longitudinal shaft of the blade, which have a threaded bore parallel to the longitudinal shaft of the blade wherein a bolt is threaded, which also traverses a sector of the hub, establishing the tightening between the hub and the blade by a nut coupled in the bolt. The use of these transversal pins constitutes an alternative solution to longitudinal inserts and is especially suitable for the case of blades with little thickness.
Document US 20070231146 has also provided the use of transversal pins, wherein the adjacent transversal pins are situated at a different distance from the root edge, thus contributing to a better distribution of stresses.
On the other hand, different geometries are known of the inserts, such as that shown in document US2012/0207607, which has a proximal end wherein a threaded bore is defined and an elongated end of practically flat wedged configuration which facilitates the lamination operation of the blade root.
The aforementioned joining solutions between hub and blade, however, can be optimized, since in this type of joints isolated stresses are generated during the blade operation which lead to problems of fatigue which affect the blade life.